1. Field of the Invention
The invention relates to a hydrometallurgical method for removing contaminants from ores and mineral concentrates. More particularly, the invention relates to the leaching of lead components from ores and mineral concentrates.
2. Description of the Art
The majority of the world's molybdenum supply originates in molybdenum sulfide mineral deposits, primarily containing molybdenum disulfide, or molybdenite. Sulfide ores normally contain their molybdenum values in very low concentrations and, therefore, require concentration procedures for eliminating the "gangue" materials. Flotation concentration is widely used to produce a molybdenum sulfide material, which can be used in lubricants or economically processed for conversion into molybdenum oxide or other marketable compounds.
Molybdenum sulfide concentrates, however, can have rather stringent requirements for purity, depending upon the ultimate use which is contemplated for the material. The large quantities of molybdenum sulfide, which are roasted to form molybdenum oxide and converted into alloys for the steel industry, are generally specified to contain no more than about 0.05 percent by weight lead. Molybdenite flotation concentrates, depending upon their mineral origin, contain about 0.1 to about 0.8 percent by weight lead, presumably present as lead sulfide. It is, therefore, necessary to provide some treatment to remove lead, either from the sulfide concentrates or from the roasted oxide products.
Most of the available methods for lead removal involve treatment with a halide substance. U.S. Pat. No. 3,674,424 to Stanley et al. teaches the use of an aqueous solution containing an alkali or alkaline earth chloride, plus either cupric or ferric chloride, as a leachant for impurity removal from molybdenite concentrates. Kentro, in U.S. Pat. No. 3,854,930, teaches lead removal from molybdenite, by leaching with hydrochloric or nitric acid, after treatment of the concentrates with ammonium chloride; U.S. Pat. No. 3,911,076 to Probert et al. is directed to a similar process. Wesely, in U.S. Pat. No. 4,083,921, heats a mixture of a chloride salt and molybdenum flotation concentrates, converting impurities into a form which is soluble in an acidic, oxidizing chloride solution.
Another method for treating molybdenite flotation concentrates is that of Queneau et al., in U.S. Pat. No. 3,834,893, wherein concentrates and sulfuric acid are roasted together, at a temperature insufficient to oxidize large amounts of molybdenum sulfide. Impurities can be leached out in two steps: first, with water and, subsequently, using hydrochloric acid. Following these steps, however, a second flotation separation is needed to recover the purified molybdenum sulfide.
Methods are known for recovering lead from its sulfide ores, including that of U.S. Pat. No. 3,933,973 to Evans et al., wherein oxygen and an acidic acetate solution react with lead sulfide, producing lead acetate and elemental sulfur. This method has been found to be ineffective for removing lead from molybdenum sulfide materials.
Lindstrom et al., in U.S. Pat. No. 3,934,070, describe a process for recovering copper from ores and ore concentrates which have adequate solubility in water. The method involves contacting the ore material with an aqueous solution of ethylenediaminetetraacetic acid ("EDTA") to dissolve copper components, and raising the solution pH to precipitate copper hydroxide. This method is said to be useful for recovering manganese, iron, cobalt, nickel, copper, and zinc from their carbonates, oxides, hydroxides, and silicates.
An abstract of a paper by Bartecki et al. (Chemical Abstracts 85:127643x) describes conditions for the recovery of copper from flotation wastes, by leaching with EDTA solutions. It is stated that EDTA leaches only copper from the wastes. Following this paper, two of its authors, L. J. Duda and Adam Bartecki, published "Dissolution of Cu.sub.2 S in Aqueous EDTA Solutions Containing Oxygen," in Hydrometallurgy, Vol. 8, pp. 341-354 (1982), to discuss the mechanism of cuprous sulfide dissolution.
Chelating agents, including EDTA, have been used to remove copper and copper compounds from steam generating equipment, as described by Harriman et al. in U.S. Pat. No. 3,438,811. Eaton et al., in U.S. Pat. No. 4,124,408, teach the removal of deposits from refrigeration system surfaces, using solutions or slurrys of EDTA.
It is clear that most of the usual techniques for lead removal from ores involve highly corrosive halide compounds. Further, such compounds are not normally easily recovered for recycle to the lead removal stages, causing economic inefficiency and difficult disposal and pollution problems. Since disposal options for toxic substances, such as lead compounds, are somewhat limited, lead typically will be concentrated and separated by neutralizing a hydrochloric acid solution; such action prevents recycling the acid. In addition, the leaching of molybdenite concentrates results in a significant solubilization of molybdenum, which is difficult to recover from solutions such as hydrochloric acid.
Accordingly, it is an object of the present invention to provide a method for removing lead from ores, using reagents which have low corrosivity toward usual construction materials.
It is a further object to provide a lead removal method which minimizes disposal and pollution problems.
Another object is to provide a method which permits the recovery of mineral values which may be solubilized during lead removal.
These and other objects will appear to those skilled in the art, from consideration of the following description and claims.